A variety of arrangements have been employed for attaching disposable, or "throwaway," cutting elements to tools used to bore, mill, shape, or otherwise process materials. These cutting elements, referred to herein as "inserts," take on a variety of shapes and forms as required to produce a desired cut in a workpiece. A typical method for securing an insert to a tool mount is to employ a threaded fastener to clamp the insert to the tool mount. One or more edge surfaces on the insert engage a complementary stop on the tool's mounting surface to prevent lateral or axial displacement of the insert during the milling operation.
Inserts having curved cutting surfaces such as used, for example, on a ball nose end mill, present special difficulty in maintaining a fixed, properly aligned connection with the tool to which they are mounted. The cutting surface of a ball nose end mill is curved relative to the axis of the rotating tool. Ball end mills initially exert an axially directed force into the tool body as the mill first engages the workpiece. Subsequently, as the bore is deepened, additional curved cutting areas of the insert are exposed to the walls of the deepening borehole. This change in both the location of contact and the amount of contact between the curved cutting surface and the wall of the bore changes the direction and magnitude of the displacing forces tending to move the insert relative to the tool mount.
Disposable inserts are periodically rotated on the ball nose end mill mount to present a new cutting edge, and worn inserts may be disposed of and replaced with new, sharp inserts. The material of the cutting element is typically substantially harder than that of the tool mount on which it is supported. For example, the inserts are frequently constructed of a tungsten carbide material, and the tool mount is frequently constructed of carbon steel. With usage, the engagement of the contacting surfaces between the tool insert and the tool mounting surface produce wear on the mounting surface that eventually permits undesired movement of the insert relative to the tool mount. The wear is accelerated as new inserts present unworn mounting surfaces for engagement with the partially worn mounting surfaces of the tool. Wear accumulates on the softer mounting surface of the tool until the mount wear becomes so severe that the tool can no longer hold the insert in proper alignment.
Inserts are held in position by providing interfering structures on the contact surfaces between the insert and the tool mount. The position fixing interface between the tool mounting surface and the insert is frequently made up of relatively complex surfaces that are difficult to fabricate. For example, it is common to employ projections extending away from the mount side of the insert and into recesses in the mounting surface of the tool to prevent relative motion between the two components. In such arrangements, it is necessary to employ one set of techniques and tools for fabricating the insert mounting surface and another set of techniques and tools for fabricating the tool mounting surface. In addition to the difficulty in manufacturing the engaging surfaces of these two components, the surface configurations are such that only a slight amount of wear in either of the surfaces permits undesired relative movement between the two components.
Prior art designs have also sought to correct the problem of insert movement by providing a clamping cap with protuberances that engage recesses in the insert and the tool body. See for example U.S. Pat. No. 4,525,110. The clamp cooperates with surfaces on the tool to precisely locate and hold the insert in a clamped cutting position. Devices of this type are complex in design and are difficult, and thus expensive, to fabricate.
Some prior art interface mounting surfaces employed between the insert and the tool mount have a single rail-like projection with a rectangular or V-shaped cross sectional profile to extend into a recess or groove with a complementary profile. See, for example, U.S. Pat. No. 5,542,795. To the extent that these structures require a relatively close fitting to prevent relative lateral motion, a rather high precision manufacturing process is required to form the elements. When a rectangular profile rail is used, some differences in size practically must exist between the projection profile and the recess profile to permit the two components to mate. This difference in size inherently frustrates the effort to fix the insert solidly to the tool body as required to resist the lateral displacement forces encountered during milling.
Another problem in the design of structures that interlock the insert and the tool mount is that the form of the interface may require the insert to be relatively thick in order to carry a complex interface design. As the insert becomes thicker, the supporting tool structure or shank must be reduced in size. As the tool shank becomes smaller, the tool is subject to an increase in chattering and tool breakage. On the other hand, if the insert is made overly thin in an effort to accommodate a complex interface structure, the insert itself is at increased risk of breaking.